Information Notice No. 95-47

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 UNITED STATES

 NUCLEAR REGULATORY COMMISSION

 OFFICE OF NUCLEAR REACTOR REGULATION

 WASHINGTON, D.C. 20555-0001

 October 4, 1995

NRC INFORMATION NOTICE 95-47: UNEXPECTED OPENING OF A SAFETY/RELIEF VALVE 

 AND COMPLICATIONS INVOLVING SUPPRESSION POOL

 COOLING STRAINER BLOCKAGE

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to a recent failure of a safety/relief valve (SRV)

to remain closed during steady-state reactor operation and the attendant

complications involving suppression pool cooling including strainer blockage. 

It is expected that recipients will review the information for applicability

to their facilities and consider actions, as appropriate, to avoid similar

problems. However, suggestions contained in this information notice are not

NRC requirements; therefore, no specific action or written response is

required.

Description of Circumstances

On September 11, 1995, the Limerick Unit 1 plant was being operated at 

100 percent power when control room personnel observed alarms and other

indications that one SRV ("M") was open. Emergency procedures were

implemented. Attempts to close the valve were unsuccessful and within 

2 minutes a manual reactor scram was initiated. The main steam isolation

valves were closed to reduce the cooldown rate of the reactor vessel. The

maximum cooldown rate was 54 øC/hr [130 øF/hr]. Before the SRV opened, the

licensee was running the "A" loop of suppression pool cooling to remove heat

being released into the pool by leaking SRVs. 

The licensee has 2-stage vertical discharge SRVs manufactured by Target Rock

Corporation. This particular valve design, which is oriented such that

condensate collects on the main stage valve seat, is believed to be the cause

of the continual problems with main stage leakage and is unique to the

Limerick units. Other licensees use Target Rock valves that have a similar 

2-stage design, but that are configured to discharge horizontally without

condensate collecting on the valve seat. Even though the licensee had

modified the valve bodies to promote drainage of condensate buildup, these

leaking valves were assumed to still have the same leakage problem through the

main stage valve seat. 

Shortly after the manual scram, and with the SRV still open, the "B" loop of

suppression pool cooling was started. Operators continued working to close 

9510030107. IN 95-47

 October 4, 1995

 Page 2 of 4

the SRV and slow the cooldown of the reactor vessel. Approximately 30 minutes

later, fluctuating motor current and flow were observed on the Unit 1 "A"

suppression pool cooling loop. Cavitation was believed to be the cause and

the loop was secured. 

After checking out the pump, the "A" pump was restarted, but at only 8 kL/m

[2000 gpm], a small fraction of the original flow. No problems were observed

so the flow rate was gradually increased to 32 kL/m [8500 gpm]. No problems

were observed so the licensee continued to operate the pump at a constant

flow. A pressure gauge located on the pump suction was observed to have a

gradually lower reading, which was believed to be indicative of an increased

pressure drop across the pump suction strainer located in the suppression

pool. After about 30 minutes of additional operation, the suction pressure

remained constant.

The rest of the reactor shutdown was routine and there were no further

complications. 

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UNITED STATES NUCLEAR REGULATORY COMMISSION
Information Notice No. 95-47

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Safety Relief Valve:

Shortly after the licensee started up following a refueling outage in March

1994, three SRVs ("F," "M," and "S") were leaking, as determined by tailpipe

temperatures which ranged from 79 øC [175 øF] to 104 øC [220 øF]. These

valves had been refurbished and reset before the restart and they were not

leaking when installed. However, SRVs "M" and "S" had been stroked during a

3550 kPa [500-psi] automatic depressurization system operability test. 

Reactor operation continued from March 1994 until September 1995, except for

two short mini-outages. Prior to the recent opening of the "M" SRV, SRVs "D"

and "L" were also observed to be leaking. Tailpipe temperatures of the five

leaking SRVs were reported as ranging between 102 øC [215 øF] to 141 øC 

[285 øF] and, because of prior experience, the leakage was believed to be past

the main stage valve seat. Tailpipes at Limerick are uninsulated. 

After the September 11, 1995 shutdown, the leaking SRVs were removed and the

"M" and "S" SRVs sent offsite for inspection to determine the root cause for

the "M" SRV opening. The "M" SRV was found to have been leaking through the

pilot valve; the other four valves were leaking through the main valve. 

Disassembly of the "M" SRV disclosed that the pilot valve disk was badly

eroded; the nose of the disk had been steam cut 360 degrees around the disk

and had separated from the rest of the disk. The interior of the disk and the

push rod also showed evidence of erosion. The pilot valve seat was eroded,

but to a lesser degree. The reason for the initial leakage is not known. The

pilot valve seat and disk were fabricated of Stellite 6 and Stellite 6B,

respectively, and were not expected to erode so severely. 

The licensee has replaced the five leaking SRVs and has resumed operation. 

The licensee is investigating techniques for identifying whether the leakage

is through the pilot valve or main stage valve. Until such a technique is. IN 95-47

 October 4, 1995

 Page 3 of 4

found, the licensee will assume that all leakage is through the pilot valve

and will monitor tailpipe temperatures so that a plan for valve replacement

will be developed by the time the tailpipe temperature exceeds 121 øC 

[250 øF]; a tailpipe temperature of 135 øC [275 øF] will cause an immediate

reactor shutdown. A steam leakage through the pilot valve of about 450 kg/hr

[1000 lb/hr] is estimated to cause a tailpipe temperature of 121 øC [250 øF]. 

Testing has demonstrated that this amount of leakage will not cause either the

pilot valve or the main stage valve to open. Actual experience at Limerick

Unit 1 showed that the "M" SRV operated for more than a year with a tailpipe

temperature in excess of 121 øC [250 øF] before it failed. The last recorded

temperature of the uninsulated tailpipe before the SRV opened was 141 øC 

[285 øF]; the temperature was recorded as 146 øC [295 øF] a week earlier. 

Suppression Pool:

Limerick Unit 1 has been in commercial operation since 1986 without having had

the suppression pool cleaned; cleaning was scheduled for the 1996 refueling

outage. The pool of Unit 2 was cleaned during the 1995 refueling outage.

After a plant cooldown following the blowdown event, a diver was sent into the

Unit 1 suppression pool to observe the condition of the strainers and 

general pool cleanliness. Each strainer is a "T" arrangement with two

truncated cones fabricated from perforated plate; the entire cone surface is

covered by a 12x12 316 L stainless steel wire mesh. The suction strainer in

the "A" loop of suppression pool cooling was found to be covered with a thin

"mat" of material, consisting of fibers and sludge. The "B" strainer had a

similar covering, but to a lesser extent. These are the two loops that had

been used for suppression pool cooling necessitated by the leaking SRVs. The

other strainers in the pool were covered with a dusting of sludge. Debris was

subsequently brushed off the surface of the strainers, and the suppression

pool floor and water were cleaned by use of a temporary filtration system. 

It is believed that, during operation of the suppression pool cooling system, 

the strainer filtered out fibers that were in the pool water. The resulting

"mat" of fibers improved the filtering action of the strainers thereby

collecting sludge and other material on the surface of the strainer. Whether

the blowdown caused by the SRV opening increased the rate of accumulation on

the strainer is not known. The licensee removed about 635 kg [1400 lb] of

debris from the pool of Unit 1. A similar amount of material had previously

been removed from the Unit 2 pool.

Analysis showed that the sludge was primarily iron oxides and the fibers were

of a polymeric nature. The source of the fibers has not been positively

identified, but the licensee has determined that the fibers were not inherent

with the suppression pool. There was no trace of either fiberglass or

asbestos fibers.. IN 95-47

 October 4, 1995

 Page 4 of 4

Related Generic Communications

 

ù NRC Information Notice 95-06: "Potential Blockage of Safety-Related

 Strainers by Material Brought Inside Containment" 

ù NRC Information Notice 93-34 and Supplement 1: "Potential for Loss of

 Emergency Core Cooling Function due to a Combination of Operational and

 Post-LOCA Debris in Containment"
Information Notice No. 95-47

file:////acfnas/perm/netsite-docs/gpo5442/in95047.html[4/18/2018 2:50:23 PM]

ù NRC Bulletin 93-02 and Supplement 1: "Debris Plugging of Emergency Core

 Cooling Suction Strainers"

ù NRC Information Notice 92-85: "Potential Failures of Emergency Core

 Cooling Systems caused by Foreign Material Blockage"

ù NRC INFORMATION NOTICE 92-71: "Partial Plugging of Suppression Pool

 Strainers at a Foreign BWR"

 

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

one of the technical contacts listed below or the appropriate Office of

Nuclear Reactor Regulation project manager.

 /s/'d by DMCrutchfield

 Dennis M. Crutchfield, Director

 Division of Reactor Program Management

 Office of Nuclear Reactor Regulation

Technical contacts: Robert Elliott, NRR

 (301) 415-1397

 Jerry Carter, NRR

 (301) 415-1153